The invention relates to a method for controlling a reductant buffer level in an exhaust gas aftertreatment device according to the preamble of the independent claim.
Present regulatory conditions in the automotive market have led to an increasing demand to improve fuel economy and reduce emissions in present vehicles. These regulatory conditions must be balanced with the demands of a consumer for high performance and quick response for a vehicle.
A diesel engine has an efficiency of up to about 52% and is thus the best converter of fossil energy. NOx emission concentration is dependent upon local oxygen atom concentration and the local temperature. Said high efficiency is however only possible at an elevated combustion temperature at which high NOx levels are inevitable. Moreover, a suppression of NOx formation by internal means (air/fuel ratio) has the tendency to cause an increase in particulates, known as the NOx-particulates trade off. Furthermore, an excess of oxygen in the exhaust gas from a diesel engine prevents the use of stoichiometric 3-way-catalyst technology for reduction of NOx as is used in gasoline engine cars from the late 80-ties.
Reducing the oxides of nitrogen (NOx) and particulate matter (PM) in exhaust gases from a diesel engine has become a very important problem in view of the protection of environment and the saving of finite fossil energy supply.
Vehicles equipped with diesel or other lean burn engines offer the benefit of increased fuel economy, however, catalytic reduction of NOx emissions via conventional means in such systems is difficult due to the high content of oxygen in the exhaust gas. In this regard Selective Catalytic Reduction (SCR) catalysts, in which NOx is continuously removed through active injection of a reductant into the exhaust gas mixture entering the catalyst are known to achieve high NOx conversion efficiency. Urea based SCR catalysts use gaseous ammonia as the active NOx reducing agent. Typically, an aqueous solution of urea is carried on board of a vehicle, and an injection system is used to supply it into the exhaust gas stream entering the SCR catalyst where it decomposes into hydro cyanic acid (NHCO) and gaseous ammonia (NH3), which is then used to convert NOx.
However, in such systems, urea injection levels have to be very precisely controlled. Under-injection of urea may result in sub-optimal NOx conversion, while over-injection may cause tailpipe ammonia slip. In a typical urea-based SCR catalyst system, the amount of urea injected is in proportion to the exhaust gas NOx concentration that represents a trade-off between maximum NOX conversion and minimum ammonia slip.
NOx conversion efficiency of an SCR catalyst is improved in the presence of adsorbed ammonia. However, it is not necessary that all of the catalyst storage capacity is utilized by ammonia in order to achieve optimal NOx conversion efficiency. On the other hand if too much ammonia is stored in the catalyst under certain operating conditions, such as high temperatures, some of the adsorbed ammonia in the catalyst may desorb and slip from the catalyst or to be oxidized to NOx and thereby reducing the overall NOx conversion efficiency.
The problem with a reductant storage catalyst is the control of the amount of reductant stored in said catalyst since a direct measurement is not possible.
It is desirable to provide an improved method for controlling a reductant buffer level in an exhaust gas aftertreatment device which keeps the system out NOx levels at a low level.
In a first aspect of the invention it is provided a method for controlling a reductant buffer level in an exhaust gas aftertreatment device connectable downstream of an internal combustion engine. Said method comprising the steps of: performing a first reductant injection of a first amount upstream said exhaust gas aftertreatment device, performing a second reductant injection of a second amount upstream said exhaust gas aftertreatment device, which second amount is different to said first amount. Said method further comprising the steps of: evaluating the NOx conversion resulting from said first and second reductant injections downstream said exhaust gas aftertreatment device to obtain a first and second result, controlling a further reductant injection in dependence of the first and second results from said first and second NOx conversion evaluations.
An advantage of the present example embodiment of the invention is that the evaluation step and the control step, forming an open loop experiment, can be added to any present control method for the buffer level in the exhaust gas after treatment system.
Another advantage of the present example embodiment of the invention is that the NOx conversion may be kept at a minimum while the open loop experiment is taking place.
In another example embodiment of the present invention a further injection amount after said evaluating step may be increased or decreased compared to said first injection if said second result from said second NOx conversion evaluation is higher or lower than said first result from said first NOx conversion evaluation given that the second injection amount was higher than the first injection amount.
In still another example embodiment of the present invention said further injection after said evaluating step may be decreased or increased compared to said first injection if said second result from said second NOx conversion evaluation is higher or lower than said first results from said first NOx conversion evaluation given that the second injection amount was lower than the first injection amount.
An advantage of these embodiment s are that small alterations of the injected reductant immediately result in a change of NOx reductant level which in turn gives an indication of a empty or full exhaust gas after treatment reductant buffer.
In still another example embodiment of the present invention said method further comprising the step of continuing said increased or decreased further injection until a predetermined level of NOx conversion is reached.
An advantage of this embodiment is that an existing buffer management method may be improved by additional method steps which are independent of said existing buffer management method.
Another advantage of this embodiment is that the buffer level can be kept at an optimal level irrespective of the starting conditions.
Still another advantage of the present invention is that the open loop experiment may be initiated as often as one may require.